。
Everyone, mind your netiquette.
Literal
Everyone netiquette [topic-は] firmly.
ネチケット is a loanword blending 'net' + 'etiquette.' しっかり 'firmly, properly' is the only predicate — no verb, just an adverb functioning as a one-word command. The sentence is a moderator's terse reminder. Structurally minimal, relying on context to fill in the implied 守ってください 'please observe.'